libmagic-sys-provided: let user override paths

let the user override assumed paths via environment.
TYPECODE_LIBMAGIC_PATH environment variable can be used to explicitly
set the path to libmagic.so. If not specified it falls back to
previously used calculation from distro data.
Location of the magic database can be specified via
TYPECODE_LIBMAGIC_DB_PATH environment variable.
If not found it falls back to the previous bevahior.

With this patch it possible to run scancode is buildsystems like YOCTO,
which heaviily rely on overriding paths via environment to pick the
correct implementation from the build workspace

Signed-off-by: Konrad Weihmann <kweihmann@outlook.com>

Author: priv-kweihmann

builtins/typecode_libmagic_system_provided/README.rst

@@ -1 +1,8 @@
-A ScanCode Toolkit plugin to use pre-installed system binary libraries and utilities.
+A ScanCode Toolkit plugin to use pre-installed libmagic library and data file
+=============================================================================
+
+the path of libmagic is either determined by distro data or explicitly
+taken from ``TYPECODE_LIBMAGIC_PATH`` environment variable. additionally
+the path to ``magic.mgc`` file can be explicitly set by using
+``TYPECODE_LIBMAGIC_DB_PATH`` environment variable, otherwise it is
+calculated from distro data

builtins/typecode_libmagic_system_provided/src/typecode_libmagic/__init__.py

@@ -22,8 +22,9 @@
 # SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 
-from os import path
 import platform
+from os import environ
+from os import path
 
 from plugincode.location_provider import LocationProviderPlugin
 
@@ -35,38 +36,46 @@ def get_locations(self):
         locations of the libmagic shared library as installed on various Linux
         distros or on FreeBSD.
         """
-        system_arch = platform.machine()
-        mainstream_system = platform.system().lower()
-        if mainstream_system == 'linux':
-            distribution = platform.linux_distribution()[0].lower()
-            debian_based_distro = ['ubuntu', 'mint', 'debian']
-            rpm_based_distro = ['fedora', 'redhat']
+        lib_dll = environ.get('TYPECODE_LIBMAGIC_PATH')
+        if not lib_dll:
+            system_arch = platform.machine()
+            mainstream_system = platform.system().lower()
+            if mainstream_system == 'linux':
+                distribution = platform.linux_distribution()[0].lower()
+                debian_based_distro = ['ubuntu', 'mint', 'debian']
+                rpm_based_distro = ['fedora', 'redhat']
+
+                if distribution in debian_based_distro:
+                    data_dir = '/usr/lib/file'
+                    lib_dir = '/usr/lib/'+system_arch+'-linux-gnu'
 
-            if distribution in debian_based_distro:
-                data_dir = '/usr/lib/file'
-                lib_dir = '/usr/lib/'+system_arch+'-linux-gnu'
+                elif distribution in rpm_based_distro:
+                    data_dir = '/usr/share/misc'
+                    lib_dir = '/usr/lib64'
 
-            elif distribution in rpm_based_distro:
-                data_dir = '/usr/share/misc'
-                lib_dir = '/usr/lib64'
+                else:
+                    raise Exception('Unsupported system: {}'.format(distribution))
 
-            else:
-                raise Exception('Unsupported system: {}'.format(distribution))
+                lib_dll = path.join(lib_dir, 'libmagic.so')
 
-            lib_dll = path.join(lib_dir, 'libmagic.so');
+            elif mainstream_system == 'freebsd':
+                if path.isdir('/usr/local/'):
+                    lib_dir = '/usr/local'
+                else:
+                    lib_dir = '/usr'
 
-        elif mainstream_system == 'freebsd':
-            if path.isdir('/usr/local/'):
-                lib_dir = '/usr/local'
-            else:
-                lib_dir = '/usr'
+                lib_dll = path.join(lib_dir, 'lib/libmagic.so')
+                data_dir = path.join(lib_dir,'share/file')
+        else:
+            lib_dir = path.dirname(lib_dll)
 
-            lib_dll = path.join(lib_dir, 'lib/libmagic.so')
-            data_dir = path.join(lib_dir,'share/file')
+        file_magic_data = environ.get('TYPECODE_LIBMAGIC_DB_PATH')
+        if not file_magic_data:
+            file_magic_data = path.join(data_dir, 'magic.mgc')
 
         locations = {
             'typecode.libmagic.libdir': lib_dir,
             'typecode.libmagic.dll': lib_dll,
-            'typecode.libmagic.db': path.join(data_dir, 'magic.mgc'),
+            'typecode.libmagic.db': file_magic_data,
             }
         return locations